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Initial characterization of interstellar comet 2I/Borisov


Interstellar comets penetrating through the Solar System had been anticipated for decades1,2. The discovery of asteroidal-looking ‘Oumuamua3,4 was thus a huge surprise and a puzzle. Furthermore, the physical properties of the ‘first scout’ turned out to be impossible to reconcile with Solar System objects4,5,6, challenging our view of interstellar minor bodies7,8. Here, we report the identification and early characterization of a new interstellar object, which has an evidently cometary appearance. The body was discovered by Gennady Borisov on 30 August 2019 ut and subsequently identified as hyperbolic by our data mining code in publicly available astrometric data. The initial orbital solution implies a very high hyperbolic excess speed of ~32 km s−1, consistent with ‘Oumuamua9 and theoretical predictions2,7. Images taken on 10 and 13 September 2019 ut with the William Herschel Telescope and Gemini North Telescope show an extended coma and a faint, broad tail. We measure a slightly reddish colour with a g′–r′ colour index of 0.66 ± 0.01 mag, compatible with Solar System comets. The observed morphology is also unremarkable and best explained by dust with a power-law size-distribution index of –3.7 ± 1.8 and a low ejection speed (44 ± 14 m s−1 for β = 1 particles, where β is the ratio of the solar gravitational attraction to the solar radiation pressure). The nucleus is probably ~1 km in radius, again a common value among Solar System comets, and has a negligible chance of experiencing rotational disruption. Based on these early characteristics, and putting its hyperbolic orbit aside, 2I/Borisov appears indistinguishable from the native Solar System comets.

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Fig. 1: Astrometric residuals of 2I/Borisov calculated for three different orbital solutions.
Fig. 2: Median-stacked images of 2I/Borisov from Gemini North.
Fig. 3: Colour of 2I/Borisov in the context of Solar System comets.

Data availability

The ACAM data are available from the corresponding authors upon reasonable request. The GMOS-N raw data will be available in the Gemini Observatory archive at after the expiration of the 12 month proprietary period.


  1. Sekanina, Z. A probability of encounter with interstellar comets and the likelihood of their existence. Icarus 27, 123–133 (1976).

    ADS  Article  Google Scholar 

  2. Engelhardt, T. et al. An observational upper limit on the interstellar number density of asteroids and comets. Astron. J. 153, 133 (2017).

    ADS  Article  Google Scholar 

  3. Williams, G. MPEC 2017-U181: Comet C/2017 U1 (PanStarrs). IAU Minor Planet Center (2017).

  4. Meech, K. J. et al. A brief visit from a red and extremely elongated interstellar asteroid. Nature 552, 378–381 (2017).

    ADS  Article  Google Scholar 

  5. Drahus, M. et al. Tumbling motion of 1I/‘Oumuamua and its implications for the body’s distant past. Nat. Astron. 2, 407–412 (2018).

    ADS  Article  Google Scholar 

  6. Micheli, M. et al. Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua). Nature 559, 223–226 (2018).

    ADS  Article  Google Scholar 

  7. The ‘Oumuamua ISSI Team. The natural history of ‘Oumuamua. Nat. Astron. 3, 594–602 (2019).

  8. Bialy, S. & Loeb, A. Could solar radiation pressure explain ‘Oumuamua’s peculiar acceleration? Astrophys. J. Lett. 868, L1 (2018).

    ADS  Article  Google Scholar 

  9. Mamajek, E. Kinematics of the interstellar vagabond 1I/‘Oumuamua (A/2017 U1). Res. Notes AAS 1, 21 (2017).

    ADS  Article  Google Scholar 

  10. Guzik, P. et al. Interstellar comet gb00234. Astronomer’s Telegram 13100 (2019);

  11. MPEC 2019-R106: Comet C/2019 Q4 (Borisov). IAU Minor Planet Center (2019).

  12. MPEC 2019-R113: Comet C/2019 Q4 (Borisov). IAU Minor Planet Center (2019).

  13. MPEC 2019-S03: Comet C/2019 Q4 (Borisov). IAU Minor Planet Center (2019).

  14. MPEC 2019-S09: Comet C/2019 Q4 (Borisov). IAU Minor Planet Center (2019).

  15. MPEC 2019-S25: Comet C/2019 Q4 (Borisov). IAU Minor Planet Center (2019).

  16. Sekanina, Z. & Kracht, R., Preperihelion outbursts and disintegration of comet C/2017 S3 (Pan-STARRS). Preprint at (2018).

  17. Fukugita, M. et al. The Sloan Digital Sky Survey photometric system. Astron. J. 111, 1748–1756 (1996).

    ADS  Article  Google Scholar 

  18. Abazajian, K. N. et al. The seventh data release of the Sloan Digital Sky Survey. Astrophys. J. Suppl. Ser. 182, 543–558 (2009).

    ADS  Article  Google Scholar 

  19. Holmberg, J., Flynn, C. & Portinari, L. The colours of the Sun. Mon. Not. R. Astron. Soc. 367, 449–453 (2006).

    ADS  Article  Google Scholar 

  20. De León, J. et al. A physical characterization of comet C/2019 Q4 (Borisov) with OSIRIS at the 10.4 m GTC. Res. Notes AAS 3, 9 (2019).

    Article  Google Scholar 

  21. Waniak, W., Zoła, S. & Krzesiński, J. Dust emission for comets Shoemaker-Levy 1991a1 and McNaught-Russell 1993v. Icarus 136, 280–297 (1998).

  22. Boe, B. et al. The orbit and size-frequency distribution of long period comets observed by Pan-STARRS1. Icarus 333, 252–272 (2019).

    ADS  Article  Google Scholar 

  23. Snodgrass, C. et al. The size distribution of Jupiter family comet nuclei. Mon. Not. R. Astron. Soc. 414, 458–469 (2011).

    ADS  Article  Google Scholar 

  24. Drahus, M. Rotational disruption of comets with parabolic orbits. In DPS Meet. 46, 200.04 (AAS, 2014).

  25. Jewitt, D. Color systematics of comets and related bodies. Astron. J. 150, 201 (2015).

    ADS  Article  Google Scholar 

  26. Solontoi, M. et al. Ensemble properties of comets in the Sloan Digital Sky Survey. Icarus 218, 571–584 (2012).

    ADS  Article  Google Scholar 

  27. Jewitt, D. The active Centaurs. Astron. J. 137, 4296–4312 (2009).

    ADS  Article  Google Scholar 

  28. Fulle, M. in Comets II (eds Festou, M. et al.) 565–575 (Univ. Arizona Press, 2004).

  29. Fulle, M., Mikuž, H. & Bosio, S. Dust environment of comet Hyakutake 1996B2. Astron. Astrophys. 324, 1197–1205 (1997).

  30. Jewitt, D. et al. Interstellar interloper 1I/2017 U1: observations from the NOT and WIYN telescopes. Astrophys. J. Lett. 850, L36 (2017).

    ADS  Article  Google Scholar 

  31. Do, A., Tucker, M. A. & Tonry, J. Interstellar interlopers: number density and origin of ‘Oumuamua-like objects. Astrophys. J. Lett. 855, L10 (2018).

    ADS  Article  Google Scholar 

  32. Moro-Martín, A., Turner, E. L. & Loeb, A. Will the Large Synoptic Survey Telescope detect extra-solar planetesimals entering the Solar System? Astrophys. J. 704, 733–742 (2009).

    ADS  Article  Google Scholar 

  33. Marsden, B. G., Sekanina, Z. & Yeomans, D. K. Comets and nongravitational forces. V. Astron. J. 78, 211–225 (1973).

  34. Cowan, J. J. & A’Hearn, M. F. Vaporization of comet nuclei: light curves and life times. Moon Planets 21, 155–171 (1979).

    ADS  Article  Google Scholar 

  35. Jorda, L., Crovisier, J. & Green, D. W. E. The correlation between visual magnitudes and water production rates. In Asteroids, Comets, Meteors 2008 1405, 8046 (LPI, 2008).

  36. Rodgers, C. T. et al. Improved u′g′r′i′z′ to UBVRCIC transformation equations for main-sequence stars. Astron. J. 132, 989–993 (2006).

  37. A’Hearn, M. F. et al. Comet Bowell 1980b. Astron. J. 89, 579–591 (1984).

  38. Weiler, M. et al. The dust activity of comet C/1995 O1 (Hale-Bopp) between 3 AU and 13 AU from the Sun. Astron. Astrophys. 403, 313–322 (2003).

    ADS  Article  Google Scholar 

  39. Campins H. & Fernández Y. Observational constraints on surface characteristics of comet nuclei. Earth Moon Planets 89, 117–134 (2002).

  40. Sekanina, Z. in Comets (ed. Wilkening, L. L.) 251–287 (Univ. Arizona Press, 1982).

  41. Samarasinha, N. H. et al. In ESA Proc. 20th ESLAB Symposium on the Exploration of Halley’s Comet Vol. 1, ESA SP-250, 487–491 (ESA, 1986).

  42. Jewitt, D. Cometary rotation: an overview. Earth Moon Planets 79, 35–53 (1997).

  43. Drahus, M. & Waniak, W. Non-constant rotation period of comet C/2001 K5 (LINEAR). Icarus 185, 544–557 (2006).

    ADS  Article  Google Scholar 

  44. Davidsson, B. J. R. Tidal splitting and rotational breakup of solid biaxial ellipsoids. Icarus 149, 375–383 (2001).

    ADS  Article  Google Scholar 

  45. SDSS photometric equations. SDSS (2005).

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Based in part on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). The William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. We thank J. Blakeslee for rapid evaluation and approval of our Gemini North director’s discretionary time request and P. Jonker for sharing time on the William Herschel Telescope. We also thank the staff of both observatories for assistance and vital contributions to making these observations possible. M.D. and P.G. are grateful for support from the National Science Centre of Poland through SONATA BIS grant no. 2016/22/E/ST9/00109 and Polish Ministry of Science and Higher Education grant no. DIR/WK/2018/12. G.C. acknowledges support from European Research Council Consolidator Grant 647208. I.P.-M. acknowledges funding from the Netherlands Research School for Astronomy (grant no. NOVA5-NW3-

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K.R. and P.G. developed Interstellar Crusher. P.G. and M.D. designed the observations, wrote the telescope time proposal, performed photometry, estimated the size of the nucleus and wrote the paper. P.G. computed the orbit. M.D. compared the colour to Solar System comets and estimated the probability of rotational disruption. W.W. reduced raw images and performed Monte Carlo dust modelling. G.C. and I.P.-M. obtained data at the William Herschel Telescope.

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Correspondence to Piotr Guzik or Michał Drahus.

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Supplementary Figs. 1–3.

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Guzik, P., Drahus, M., Rusek, K. et al. Initial characterization of interstellar comet 2I/Borisov. Nat Astron 4, 53–57 (2020).

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